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Olfactory memory

Olfactory memory encompasses the cognitive processes by which individuals encode, store, and retrieve information about odors and the experiences or emotions associated with them, often evoking vivid autobiographical recollections known as the Proust phenomenon.^[1] Unlike memories from other sensory modalities, olfactory memories are particularly potent due to the olfactory system's direct anatomical connections to the limbic structures, including the amygdala and hippocampus, bypassing the thalamus and facilitating rapid emotional processing.^[2] This results in odors triggering spontaneous, emotionally charged memories that are more vivid and arousal-linked compared to visual or verbal cues.^[2] Key properties of olfactory memory include its representational nature, where learning progressively refines neural transformations of odor information, and its persistence through mechanisms like synaptic plasticity and adult neurogenesis in the olfactory bulb.^[3] Formation involves distinct phases: short-term memory relies on immediate synaptic changes, such as NMDA receptor-dependent long-term potentiation, while long-term consolidation requires protein synthesis cascades involving PKA, CREB, and BDNF, often spanning hours to days.^[3] Appetitive olfactory learning, such as associating odors with food rewards, typically builds gradually through statistical patterns, contrasting with the one-trial efficiency of aversive conditioning, like fear responses to threatening smells.^[3] The primary neural substrates include the olfactory bulb for initial processing and pattern separation, the piriform cortex for associative encoding, and the entorhinal-hippocampal network for spatial and contextual integration, with the amygdala modulating emotional valence.^[3] In humans and rodents, these pathways support developmental shifts, from neonatal attachment learning—where maternal odors guide survival—to adult fear modulation, where newborn neurons in the olfactory bulb help reconcile memory stability with flexibility.^[4] Disruptions, such as in neurodegenerative diseases, highlight olfactory memory's sensitivity, as early olfactory deficits often precede broader cognitive decline.^[2]

Neural and Physiological Mechanisms

Olfactory Detection and Processing

The olfactory epithelium, located in the nasal cavity, is a specialized pseudostratified neuroepithelium that serves as the primary site for odor detection. It consists of several cell types, including olfactory receptor neurons (ORNs), which are bipolar neurons with cilia extending into the mucus layer covering the epithelium. These ORNs detect odorant molecules—volatile chemical compounds that dissolve in the nasal mucus and bind to specific receptors on the ciliary surface.^[5]^[6] Upon binding, odorant molecules interact with G-protein-coupled receptors (GPCRs) expressed on the ORN cilia, initiating the signal transduction pathway. This binding activates a specific G protein (Golf in vertebrates), which stimulates adenylate cyclase type III to produce cyclic adenosine monophosphate (cAMP). The increased cAMP levels open cyclic nucleotide-gated (CNG) ion channels, allowing influx of Na⁺ and Ca²⁺ ions, leading to depolarization of the ORN membrane and generation of action potentials. This process converts chemical signals into electrical impulses that propagate along the ORN axon to the olfactory bulb.^[7]^[8] Axons from ORNs converge in glomeruli within the olfactory bulb, where they synapse with mitral and tufted cells, forming the first central relay station for olfactory information. From the olfactory bulb, processed signals project directly to the primary olfactory cortex, particularly the piriform cortex, without thalamic intermediation, and integrate with limbic structures such as the entorhinal cortex and amygdala, facilitating early associative tagging for potential memory formation.^[9]^[10] Neuromodulators play a crucial role in modulating olfactory detection to enhance signal salience, particularly during novel or behaviorally relevant exposures. Acetylcholine, released from centrifugal fibers originating in the basal forebrain, increases excitability in the olfactory bulb and piriform cortex, sharpening odor discrimination. Serotonin, from raphe nuclei projections, fine-tunes glomerular activity to adapt to stimulus intensity, while norepinephrine, from locus coeruleus inputs, boosts overall signal-to-noise ratios during arousal states, thereby prioritizing salient odors for further processing.^[11]^[12] In humans, approximately 400 functional olfactory receptor genes encode these GPCRs, enabling detection of a vast array of odorants, which are often classified by molecular structure into categories such as aldehydes (e.g., contributing to sharp, metallic scents) and esters (e.g., evoking fruity notes).^[13]

Implicit Odor Memory

Implicit odor memory in olfaction refers to non-declarative forms of memory that function without conscious awareness, primarily through procedural mechanisms such as priming, habituation, and perceptual learning, distinguishing it from explicit memory that involves deliberate recollection of odors.^[14] This type of memory enables automatic behavioral and neural adaptations to olfactory stimuli, facilitating efficient processing of environmental cues without requiring cognitive effort.^[15] A key manifestation of implicit odor memory is habituation, characterized by a progressive decrease in neural and behavioral responses to repeated or prolonged exposure to the same odor, allowing the olfactory system to filter out irrelevant or predictable stimuli.^[14] This process is mediated by short-term synaptic depression at synapses involving mitral cells in the olfactory bulb, where repeated activation reduces neurotransmitter release, often via dopamine D2 receptors, leading to diminished output from mitral and tufted cells.^[14] Studies indicate that habituation onset occurs rapidly, within seconds to minutes of exposure—for instance, short-term habituation following 20-second odor presentations can last approximately 2 minutes—while cross-habituation extends to structurally similar odors, such as vanillin and ethyl vanillin, due to overlapping neural representations.^[16]^[17] Perceptual learning represents another unconscious aspect of implicit odor memory, where repeated exposure enhances the ability to discriminate between similar odors without intentional training, thereby sharpening olfactory acuity over time.^[14] This improvement arises from experience-dependent plasticity in the piriform cortex, particularly the anterior region, where synaptic changes, including long-term potentiation at association fiber synapses modulated by acetylcholine, refine odor object representations and reduce adaptation to familiar stimuli.^[14] Such plasticity supports subtle perceptual enhancements, as demonstrated in human neuroimaging studies showing modified odor-evoked activity in piriform regions following prolonged exposure.^[18] In animal models, particularly rodents, implicit odor memory is evidenced by reduced sniffing and investigative behaviors toward familiar odors, reflecting habituation at both neural and behavioral levels.^[19] For example, in habituation-dishabituation paradigms, mice exhibit decreased investigation time to repeatedly presented odors, with recovery upon introduction of a novel stimulus, underscoring the specificity and reversibility of these unconscious adaptations mediated by olfactory bulb circuitry.^[20] This behavioral metric highlights the ecological role of implicit memory in prioritizing novel olfactory signals for survival.^[19]

Explicit Odor Memory

Explicit odor memory refers to the conscious, declarative recall of odors, involving hippocampal-dependent processes that enable recognition and identification with awareness of contextual details. This form of memory allows individuals to deliberately retrieve information about previously encountered odors, distinguishing it from automatic or non-conscious processes. The hippocampus, along with surrounding medial temporal lobe structures such as the entorhinal cortex, plays a central role in forming and retrieving these episodic representations, integrating odor percepts with temporal and spatial context.^[21] Odor recognition, a key component of explicit odor memory, involves judging the familiarity of an odor without necessarily naming it, often tested through tasks where participants identify previously exposed scents from novel ones. Human studies show recognition accuracy around 73% immediately following brief exposure to common odors, outperforming chance and remaining detectable even after extended delays, such as 64 days, where performance stays above chance levels (d' ≈ 0.50 for less familiar odors). This accuracy is influenced by factors like odor familiarity and semantic processing, with higher rates for well-known scents due to stronger hippocampal encoding. Verbal interference minimally affects recognition, highlighting odors' relative independence from linguistic mediation compared to other sensory modalities.^[21]^[22] Odor identification extends explicit memory to semantic labeling, where individuals assign verbal or conceptual names to odors, a process that engages prefrontal and temporal regions alongside the hippocampus for associative retrieval. Accuracy for identification is generally lower than for pure recognition, often around 50-60% for familiar odors in standard tests, due to the limited vocabulary for odors and reliance on contextual cues. For instance, consistent naming predicts near-perfect recognition (hit rates up to 97%), but inconsistent or incorrect labeling leads to poorer performance, underscoring the role of verbalization in explicit recall. This lower accuracy compared to visual memory (where identification exceeds 80% for familiar items) stems from odors' abstract, less categorical nature, making semantic access more effortful.^[21]^[22] In learning odor lists, explicit memory exhibits primacy and recency effects akin to those in verbal memory tasks, with better recall for items at the beginning (primacy, due to deeper encoding) and end (recency, from short-term maintenance) of sequences. For example, recall of odor names shows primacy under verbal elaboration conditions and recency across both elaborated and non-elaborated presentations, reflecting serial position influences similar to word lists. Odors demonstrate higher resistance to interference than verbal materials; retroactive interference from intervening stimuli has minimal impact on odor recognition, allowing sustained performance over retention intervals where word memory declines more sharply. This resilience is evident in short-term tasks, where odor memory remains stable despite distractors, attributed to configural encoding in olfactory pathways. Experimental paradigms for assessing explicit odor memory, such as delayed matching-to-sample (DMTS) tasks, involve presenting a sample odor followed by a delay and then target and distractor options, requiring conscious judgment of matches. In humans, DMTS reveals that explicit odor memory decays more gradually over long delays (e.g., months) than expected for other senses, with performance holding above chance due to hippocampal consolidation, though slower than the persistent implicit adaptations in perceptual learning. These tasks integrate olfactory inputs via the piriform cortex to hippocampal circuits for contextual binding, supporting deliberate retrieval.^[21]

Key Brain Regions and Pathways

The core neural pathway for olfactory memory encoding begins in the olfactory bulb, where initial odor signals are processed and relayed directly to the piriform cortex for associative learning and representation.^[23] From the piriform cortex, projections extend to the entorhinal cortex, which integrates olfactory input with spatial and contextual information, before converging on the hippocampus to facilitate long-term consolidation of odor-related episodic memories.^[24] This trisynaptic pathway—olfactory bulb to piriform cortex, entorhinal cortex, and hippocampus—enables the binding of odor percepts with temporal and autobiographical contexts, distinct from other sensory modalities due to its direct limbic access without thalamic relay.^[25] The amygdala plays a pivotal role in emotionally tagging odors, modulating memory consolidation by enhancing synaptic plasticity in connected regions during states of arousal.^[20] Specifically, noradrenergic inputs from the locus coeruleus to the basolateral amygdala during emotional events strengthen odor-hippocampal interactions, prioritizing salient odors for long-term storage via beta-adrenergic receptor activation.^[20] Post-2010 fMRI studies have demonstrated that amygdala activation during odor exposure correlates with the intensity of subsequent fear memory retrieval, particularly in conditioning paradigms where odors serve as conditioned stimuli for aversive outcomes.^[26] Hemispheric asymmetries influence olfactory memory processing, with the right hemisphere showing dominance in encoding odor intensity and emotional valence, while the left hemisphere excels in odor identification and verbal-semantic associations.^[27] Lesion studies confirm this lateralization; for instance, damage to the right temporal lobe, including amygdala structures, selectively impairs recall of emotionally charged odors, whereas left-sided lesions disrupt naming and familiarity judgments more profoundly.^[28] These differences arise from asymmetric connectivity, where right piriform-amygdala pathways prioritize affective processing and left entorhinal-hippocampal links support declarative aspects.^[29] Beyond the primary pathway, the orbitofrontal cortex integrates olfactory signals with reward valuation, assigning hedonic and motivational significance to odors during memory formation.^[30] This region modulates odor preferences by representing expected outcomes, facilitating adaptive behaviors like avoidance of spoiled food.^[30] Theta oscillations (4-8 Hz) further synchronize olfactory-hippocampal interactions, emerging during odor sniffing to coordinate phase-locked activity between piriform cortex and anterior hippocampus, thereby supporting rapid encoding of odor-specific content.^[31]

Behavioral and Cognitive Effects

Emotional and Autobiographical Recall

The Proust phenomenon refers to the potent ability of odors to trigger involuntary autobiographical memories, often more vividly and emotionally than cues from other sensory modalities, owing to the olfactory system's direct neural projections to the limbic system, including the amygdala.^[32] This effect, named after Marcel Proust's description in In Search of Lost Time, underscores how scents can evoke detailed recollections of personal events without conscious effort, bypassing typical cortical processing routes that dilute emotional intensity in visual or verbal recall.^[33] Odor-evoked memories are particularly influenced by emotional valence, where positive scents like lavender promote associations with calmness and serenity, while negative ones such as rotten eggs elicit disgust and aversion, thereby amplifying the specificity and intensity of recalled episodes.^[34] These hedonic associations enhance memory congruence, as demonstrated in experiments where emotionally matched odors (e.g., pleasant scents paired with positive experiences) led to stronger retrieval of affective details compared to mismatched cues.^[35] In laboratory settings, such as those conducted by Rachel Herz in the 1990s and 2010s, odor-cued memories were rated as significantly more emotional than visual cues, with participants reporting heightened arousal and personal relevance.^[32] Cognitively, these memories exhibit enhanced episodic detail, longer subjective duration, and superior vividness, often described as rarer and more specific than those triggered by other senses, fostering a deeper sense of reliving the original event.^[34] Cultural variations further shape these odor-emotion links, with cross-cultural studies revealing differences in how scents are associated with affective states; for instance, certain odors deemed pleasant in Western contexts may evoke neutral or negative responses in Asian cultures due to experiential and semantic influences.^[36] Recent advancements post-2020 have explored virtual reality (VR) integrated with odor delivery for therapeutic memory recall, showing that multisensory VR environments—combining scents with immersive visuals and sounds—significantly boost immersion, reduce stress, and elevate optimism during autobiographical retrieval compared to odor-only methods.^[37] Such approaches hold promise for clinical applications, like PTSD treatment, by leveraging odors to reframe traumatic memories through controlled exposure.^[38]

Influence on Mood and Stress

Olfactory memories can significantly enhance mood through the activation of pleasant odor associations, often formed via classical conditioning, leading to increased positive affect. In human trials, exposure to conditioned pleasant odors has been shown to elevate scores on the Positive and Negative Affect Schedule (PANAS), with participants reporting heightened feelings of enthusiasm and alertness compared to neutral or unpleasant odor conditions.^[39] This effect stems from implicit associations linking odors to rewarding past experiences, which bypass conscious recollection to directly influence emotional valence. Stress reduction is another key influence of olfactory memories, where odor-cued recall activates parasympathetic nervous system responses, resulting in decreased cortisol levels following exposure. Studies demonstrate that familiar comforting odors, such as those evoking positive childhood memories, can lower salivary cortisol in acute stress paradigms, promoting relaxation without deliberate memory retrieval. These autonomic shifts are mediated by limbic pathways, including the amygdala and hippocampus, where implicit odor traces modulate arousal independently of explicit awareness.^[40] Aromatherapy research from 2015 to 2024 highlights lavender odor's role in reducing anxiety in clinical settings, as measured by the State-Trait Anxiety Inventory (STAI), particularly in patients undergoing medical procedures.^[41] This anxiety alleviation is attributed to olfactory memories reinforcing calming associations, enhancing overall mood stability. Sex differences have also been noted, with women exhibiting stronger stress-reducing responses to odors, potentially due to variations in olfactory bulb connectivity and hormonal influences on emotional processing.^[42] Recent 2020s research explores olfactory memory integration in virtual environments for mood regulation in post-traumatic stress disorder (PTSD), where personalized odor cues paired with immersive scenarios have reduced hyperarousal symptoms by facilitating adaptive emotional reconsolidation. These interventions leverage implicit memory to dampen fear responses, showing preliminary efficacy in reducing PTSD symptoms during exposure therapy.^[38]

Developmental Roles

Olfactory memory plays a crucial role in maternal bonding from the earliest stages of life, enabling newborns to recognize and prefer their mother's scent shortly after birth. Human infants demonstrate an attraction to maternal odors, such as those from amniotic fluid or breast milk, within minutes to hours, which facilitates initial attachment and orientation toward the nipple during breastfeeding. This preference is mediated by implicit olfactory memory processes involving the amygdala-hippocampal circuits, which support unconscious recognition without requiring explicit recall.^[43]^[44]^[43] In early neurological development, olfactory cues contribute to cortical plasticity and synapse formation, shaping brain growth in infancy. Exposure to environmental odors during critical periods promotes dendritic growth in projection neurons and enhances synaptic connections within olfactory glomeruli, as observed in rodent models where early sensory experiences refine neural circuits. These processes extend to broader brain maturation, influencing the integration of sensory inputs that underpin cognitive and emotional foundations.^[45]^[45] Early odor exposures have long-term implications for cognitive and emotional development, aligning with attachment theory by fostering secure bonds that predict later relational outcomes. In mammalian studies, such as those with rat pups, pairing maternal odors with the mother enhances survival through improved nest orientation and reduced stress responses, demonstrating adaptive benefits. Human research from the 2010s links familiarity with amniotic fluid or breast milk odors to successful breastfeeding transitions in preterm infants, correlating with better feeding behaviors and attachment security. Cross-species evidence from primates, including chimpanzees, shows olfactory imprinting aids kin and group recognition, supporting social hierarchies and cohesion in early development.^[46]^[47]^[48]

Evolutionary Perspectives

Foraging and Survival

Olfactory memory plays a crucial role in foraging by enabling animals to remember and follow odor trails associated with food sources, a mechanism conserved across species from insects to mammals. In insects like Drosophila melanogaster, associative learning allows flies to form long-term memories linking specific odors to rewarding food, guiding efficient navigation to nutrient-rich locations during foraging. For instance, studies on fruit flies demonstrate that odor-sugar associations persist for hours to days, facilitating repeated visits to food sites and enhancing survival through optimized energy acquisition. This capacity is similarly evident in mammals, where olfactory memory supports tracking of prey scents over extended periods; gray wolves (Canis lupus), for example, rely on memorized olfactory cues to pursue ungulate trails spanning multiple days, integrating scent persistence with spatial recall to locate elusive prey in vast territories.^[49]^[50]^[51] In addition to food location, olfactory memory aids survival by promoting rapid aversion to warning stimuli, such as toxic odors, through implicit learning processes that bypass conscious recall. Animals quickly form conditioned aversions to odors signaling danger, like bitter alkaloids in plants, which indicate potential toxicity and deter ingestion. In insects, such as honeybees, pairing an odor with a toxin induces a robust olfactory aversion that strengthens over time, reducing exposure to harmful substances and improving foraging safety. This rapid implicit memory for aversive odors is conserved in mammals, where it enables quick recognition and avoidance of contaminated food sources, thereby minimizing physiological risks during resource acquisition.^[52]^[53] The adaptive value of olfactory memory in foraging and survival is underscored by its genetic foundations, particularly in the evolution of olfactory receptors tuned for detecting and memorizing cues related to predator avoidance. Vertebrate olfactory receptor genes have diversified to encode sensitivity to specific odorants, such as predator-derived sulfides, allowing prey species like mice to innately associate and memorize these scents with danger, facilitating evasion behaviors. This genetic architecture supports memory-based predator avoidance, where learned olfactory associations enhance survival rates by integrating sensory detection with behavioral responses.^[54]^[55] Fossil evidence highlights the evolutionary significance of olfactory memory for survival in early hominins, as indicated by variations in olfactory bulb size. Early hominins, such as Australopithecus and Homo erectus, exhibited relatively larger olfactory bulbs compared to modern humans, correlating with ecological pressures where scent-based memory was vital for locating resources and avoiding predators in diverse habitats. In contrast, analyses of Neanderthal endocasts suggest olfactory bulbs approximately 12% smaller relative to brain size compared to modern humans.^[56]^[57] Olfactory memory facilitates social communication in vertebrates primarily through implicit processing of pheromones, enabling kin recognition and territory marking. In rodents and other mammals, individuals memorize familial scents via phenotype matching, a mechanism where self-referent odors are compared to those of others to distinguish relatives and prevent inbreeding.^[58] This process is implicit and relies on the main olfactory system, with studies in Belding's ground squirrels demonstrating that juveniles learn maternal and sibling odors within days of emergence, using them for selective social interactions.^[59] Territory marking further leverages this memory, as animals deposit urine or glandular scents that conspecifics recognize and recall to assess ownership, dominance, or intrusion risks, reducing aggressive encounters in species like mice and carnivores.^[60] In reproductive contexts, olfactory memory underpins mate selection by encoding preferences for scents linked to major histocompatibility complex (MHC) dissimilarity, promoting genetic diversity in offspring. Rodent models, such as mice, show females memorizing and preferring male urinary odors indicating dissimilar MHC types, a process mediated by vomeronasal organ detection and olfactory bulb plasticity.^[61] In humans, seminal research from the 1990s revealed that women, particularly those not using hormonal contraceptives, rate body odors from MHC-dissimilar men as more pleasant, suggesting an evolved, subconscious memorization of compatibility cues despite reduced olfactory reliance compared to other primates.^[62] Later studies confirmed this pattern across diverse populations, linking it to immune gene optimization, though preferences can vary with context like oral contraceptive use.^[63] Olfactory memory also supports individual odor identification and social bonding in group-living animals, where pack members learn unique scent signatures for affiliation and cooperation. In prairie voles, a monogamous rodent model, cohabitation with a partner leads to implicit memorization of their pheromones, forming selective pair bonds that persist for weeks and manifest as partner preference and aggression toward intruders.^[64] This bonding involves oxytocin-modulated plasticity in the olfactory bulb and amygdala, with disruption of olfactory input impairing bond formation.^[65] Similar mechanisms occur in sheep, where ewes rapidly memorize lamb odors post-partum for exclusive nursing, reinforced by hormonal changes and main olfactory pathways. Evolutionarily, olfactory memory's social and reproductive roles are conserved across vertebrates but diminished in humans due to the ascendancy of visual and auditory cues in mate choice and group dynamics. While non-human mammals depend heavily on scent for these functions, human studies indicate retention in subtle, subconscious processes like initial attraction to body odors, reflecting a phylogenetic legacy amid sensory trade-offs.^[66]

Clinical Aspects and Deficits

Associated Neurological Disorders

Olfactory memory deficits are closely linked to various neurological disorders, particularly neurodegenerative conditions where early sensory impairments signal underlying pathology in olfactory processing regions. In Parkinson's disease (PD), hyposmia affects approximately 90% of patients and often emerges as one of the earliest non-motor symptoms due to alpha-synuclein aggregation in the olfactory bulb, disrupting odor encoding and recall processes.^[67] According to Braak staging, this pathology begins in the olfactory structures during stage 1, preceding involvement of motor-related areas and highlighting the olfactory system's vulnerability in disease propagation.^[67] In Alzheimer's disease (AD), olfactory dysfunction occurs in up to 90% of cases, stemming from amyloid-beta plaques and tau tangles in the olfactory bulb and anterior olfactory nucleus, which impair memory consolidation for odors and contribute to broader cognitive decline.^[68] Mental illnesses also exhibit notable olfactory memory impairments. In schizophrenia, patients demonstrate significant deficits in odor identification, with meta-analyses revealing large effect sizes (Cohen's d ≈ 0.93).^[69] Depression is associated with olfactory anhedonia, where individuals show reduced responsiveness to pleasant odors, affecting hedonic memory and emotional associations with scents, as evidenced by impaired discrimination of odor concentrations in clinical studies.^[70] Structural brain changes further underpin these deficits, with atrophy in the piriform cortex and amygdala correlating strongly with olfactory memory loss across disorders like PD and AD.^[71] These regions, integral to odor perception and emotional tagging, exhibit volume reductions that predict the severity of memory impairments. Recent investigations into COVID-19 (2020-2025) reveal that persistent parosmia in 20–50% of survivors disrupts olfactory memory, with scoping reviews linking such dysfunction to broader cognitive deficits, including episodic memory decline.^[72] Overall, olfactory loss holds predictive value as a biomarker, particularly in PD, where it can precede motor symptoms by 5-10 years, enabling earlier detection of at-risk individuals.^[73]

Diagnostic Testing

Diagnostic testing for olfactory memory impairments primarily involves standardized psychophysical assessments that evaluate explicit recognition and identification of odors, as well as implicit processes through physiological responses. The University of Pennsylvania Smell Identification Test (UPSIT) is a widely used tool for assessing explicit olfactory memory, consisting of a 40-item scratch-and-sniff booklet where participants identify odors from multiple-choice options, providing a score out of 40 that reflects recognition memory performance.^[74]^[75] Scores below 34 on the UPSIT are indicative of significant olfactory deficits, particularly when combined with self-reported anosmia, offering a specificity of 64% for early detection in neurological contexts.^[76] Another common test, the Sniffin' Sticks, employs odor-impregnated pens to measure olfactory thresholds, discrimination, and identification, allowing for the evaluation of memory-related aspects such as odor recognition and differentiation.^[77]^[78] This test is particularly valuable for explicit memory assessment in clinical settings, as it quantifies the ability to recall and distinguish previously encountered odors. Norms for these tests are adjusted for age and sex, with olfactory function declining progressively with age and generally higher performance in females, ensuring accurate interpretation of results.^[79]^[80] For implicit olfactory memory, habituation paradigms are employed, where repeated exposure to an odor leads to a decrement in neural or physiological responses, measurable via electroencephalography (EEG) or functional magnetic resonance imaging (fMRI). These methods detect subconscious familiarity without requiring verbal recall, revealing response reductions in olfactory cortices after prolonged stimulation.^[81]^[82] In clinical protocols, olfactory function is often scored using the threshold-detection-discrimination-identification (TDI) composite from tools like Sniffin' Sticks, with a maximum score of 48; scores below 30.75 typically indicate dysfunction, adjusted for demographic factors to guide diagnosis in conditions such as post-viral anosmia.^[83]^[84] Recent applications include post-COVID-19 evaluations, where testing reveals persistent impairments in up to 80% of affected individuals even years after infection, though recovery rates remain low without intervention, with some studies reporting partial restoration in 10-20% of cases by 2025 through serial assessments.^[85]^[72] Advances in digital olfactometers enable precise, controlled odor delivery for memory tasks, integrating hardware for automated presentation and remote testing to enhance accuracy in both explicit identification and implicit habituation studies.^[86]^[87]

Therapeutic Interventions

Olfactory training, a non-invasive intervention involving daily exposure to specific odors such as rose, lemon, eucalyptus, and clove for approximately 20 seconds each twice daily over several months, aims to rebuild neural circuits associated with olfactory memory and perception.^[88] This approach has demonstrated efficacy in 30-50% of cases of persistent post-viral olfactory dysfunction, with recovery rates around 36% in related post-traumatic scenarios and many post-COVID-19 patients reporting subjective improvements after 4-12 weeks of training.^[89] Protocols developed between 2020 and 2025 for COVID-19-related anosmia emphasize consistent training, resulting in significant enhancements in odor identification scores, with meta-analyses showing mean differences of 2-5 points on standardized tests like the UPSIT compared to controls.^[90] Cultural adaptations enhance accessibility by substituting familiar local odors, such as those used in Arabic-modified kits, to improve patient adherence and relevance.^[91] Pharmacological interventions target underlying mechanisms of olfactory memory deficits in neurological conditions. In Parkinson's disease (PD), where alpha-synuclein aggregation contributes to early olfactory loss, inhibitors like prasinezumab are in phase III clinical trials to reduce pathology, with preclinical models showing amelioration of deficits through related compounds such as rasagiline.^[92]^[93] For depression-linked anosmia, selective serotonin reuptake inhibitors (SSRIs) and similar neuromodulators, including fluoxetine and venlafaxine, enhance olfactory function by restoring deficits associated with mood disorders, with studies reporting normalization in patients post-remission and positive responses in two-thirds of post-COVID cases.^[94]^[95] Emerging therapies integrate technology and regenerative approaches to rehabilitate olfactory memory. Virtual reality (VR) systems combined with odor delivery have shown promise in Alzheimer's disease by stimulating memory-related brain regions during immersive tasks, leading to significant improvements in visuospatial memory (p=0.024) and cognitive processing in older adults.^[96] Stem cell therapies, particularly intranasal transplantation of neural stem cells, promote regeneration of the olfactory epithelium in animal models of anosmia, increasing epithelial thickness by up to 200% and enhancing functional recovery as measured by behavioral tests.^[97] Longitudinal studies indicate that early olfactory training interventions can delay disease progression in conditions like mild cognitive impairment and PD by preserving olfactory function, which correlates with slower cognitive decline and reduced risk of dementia (hazard ratio 1.90-2.48 in at-risk groups).^[98] These outcomes underscore the potential of targeted olfactory memory restoration to mitigate broader neurological deterioration.^[99]

References

[1]
Odor memory: Review and analysis | Psychonomic Bulletin & Review
We critically review the cognitive literature on olfactory memory and identify the similarities and differences between odor memory and visual-verbal memory.
[2]
Memory and Plasticity in the Olfactory System: From Infancy ... - NCBI
One of the most characteristic features of odor memory in humans is the rather unique ability of odors to vividly trigger the evocation of emotional ...INTRODUCTION · EMOTIONAL ODOR MEMORY... · DEVELOPMENT OF ODOR...
[3]
Properties and mechanisms of olfactory learning and memory - PMC
Consolidation of an olfactory memory trace in the olfactory bulb is required for learning-induced survival of adult-born neurons and long-term memory. PLoS ...
[4]
Olfactory neurogenesis and its role in fear memory modulation
Sep 27, 2023 · Olfactory memory alludes to the memory of odors and the memory associated with odors (Herz and Engen, 1996). Therefore, a perceived odor ...
[5]
The Olfactory Epithelium and Olfactory Receptor Neurons - NCBI - NIH
Structure and function of the olfactory epithelium. (A) Diagram of the olfactory epithelium showing the major cell types: olfactory receptor neurons and their ...Missing: ORNs binding
[6]
The Olfactory System: Basic Anatomy and Physiology for General ...
An odorant molecule binds to an olfactory receptor neuron, generating an action potential that is relayed to the olfactory bulb and then to the olfactory cortex ...
[7]
Mammalian olfactory receptors: pharmacology, G protein coupling ...
In this review, we will discuss recently published studies outlining the molecular basis of odor discrimination, focusing on pharmacology, G protein activation ...
[8]
Ric-8B, an Olfactory Putative GTP Exchange Factor, Amplifies Signal ...
Apr 13, 2005 · The activated receptors couple to an olfactory-specific G-protein (Golf), which activates adenylyl cyclase III to produce cAMP. Increased cAMP ...
[9]
Olfactory Information Storage Engages Subcortical and Cortical ...
Aug 11, 2021 · The olfactory bulb (OB) delivers sensory information to the piriform cortex (PC) and other components of the olfactory system.
[10]
The Bulb, the Brain and the Being: New Insights into Olfactory ...
Thus, the secondary olfactory areas consist partly of limbic and paralimbic brain areas that are involved in regulating behavior, memory and cognition. Numerous ...
[11]
The Neuromodulatory Role of the Noradrenergic and Cholinergic ...
Jul 7, 2022 · There are four primary neuromodulatory systems: acetylcholine, norepinephrine, dopamine, and serotonin. Each of these four molecules plays ...
[12]
Neural correlates of olfactory learning: Critical role of centrifugal ...
Both the olfactory bulb and piriform cortex receive input from multiple neuromodulatory regions releasing acetylcholine, norepinephrine, and serotonin.
[13]
Update on the olfactory receptor (OR) gene superfamily - PMC - NIH
The olfactory receptor gene (OR) superfamily is the largest in the human genome. The superfamily contains 390 putatively functional genes and 465 pseudogenes.
[14]
Plasticity in the Olfactory System: Lessons for the Neurobiology of ...
Implicit Memory. Implicit memory includes habituation, sensitization, classical conditioning, and perceptual learning (Clark and others 2002). Habituation is a ...
[15]
Why human olfaction should not be modeled on theories and tasks ...
Sep 27, 2023 · We claim that implicit memory plays a key role in olfaction and that explicit recollection/memory of an odor does not have much ecological ...
[16]
The Habituation/Cross-Habituation Test Revisited - PubMed Central
The habituation/cross-habituation test (HaXha) is a spontaneous odor discrimination task that has been used for many decades to evaluate olfactory function ...Missing: vanillin ethyl
[17]
Distinct neural mechanisms mediate olfactory memory formation at ...
We demonstrate two neural mechanisms underlying simple olfactory learning, distinguished by their persistence and specificity, mediated by different olfactory ...
[18]
Experience-Dependent Neural Plasticity in Human Piriform and ...
Dec 21, 2006 · Recent electrophysiological studies in anesthetized rats suggest that perceptual learning modifies odor-evoked activity in piriform cortex ...Results · Experimental Procedures · Supplemental Data
[19]
Testing for Odor Discrimination and Habituation in Mice - PMC - NIH
May 5, 2015 · A typical mouse shows a decrease in response to the odor over repeated presentations (habituation). The experimenter then presents a novel odor ...
[20]
Noradrenergic Induction of Odor-Specific Neural Habituation and ...
Oct 15, 2008 · Such familiar stimuli elicit reduced behavioral responses, a change likely related to NA-dependent plasticity in the olfactory bulb (OB). In ...
[21]
https://doi.org/10.3389/fnbeh.2014.00240
[22]
https://doi.org/10.1093/chemse/bjv008
[23]
Properties and mechanisms of olfactory learning and memory
The proliferation of new neurons ceases prior to adulthood in most brain regions, with the exception of the hippocampus and OB, and possibly the hypothalamus ( ...
[24]
Characterizing functional pathways of the human olfactory system
Jul 24, 2019 · The brain areas that showed connectivity unique to the anterior olfactory nucleus were largest in the orbitofrontal cortex, with smaller ...
[25]
Olfactory memory networks: from emotional learning to social ... - PMC
Feb 17, 2015 · Recent work has begun exploring the role of sleep in olfactory memory. Barnes and Wilson (2014) reviewed recent evidence of sleep-dependent ...
[26]
Disruption of Reconsolidation Erases a Fear Memory Trace in the ...
Using functional brain imaging, we found that, after a conditioned fear memory was formed, reactivation and reconsolidation left a memory trace in the ...<|separator|>
[27]
Hemispheric Differences in Odour Recognition - PubMed
Two experiments were designed in order to study hemispheric differences in odour recognition in normal subject. In the first experiment subjects first ...
[28]
Odor Recognition Memory in Humans: Role of Right Temporal and ...
The results showed impairment only after excision from the right temporal or right orbitofrontal cortex. All groups showed significant forgetting over time, and ...
[29]
A review on the neural bases of episodic odor memory - Frontiers
For instance, Larsson et al. (2006) showed that recognition is more based on recollection than familiarity for familiar odors, and is more based on familiarity ...<|separator|>
[30]
The Orbitofrontal Cortex Is Required for Learned Modulation of ...
Oct 15, 2024 · The orbitofrontal cortex (OFC) is a fourth-order olfactory brain area, involved in flexible value association, with behaviorally relevant ...
[31]
Theta Oscillations Rapidly Convey Odor-Specific Content in Human ...
Theta coupling between piriform cortex and hippocampus emerges in presence of odor. While odor-driven theta oscillations in human PPC appear to support rapid ...
[32]
A naturalistic analysis of autobiographical memories triggered by ...
Results revealed that memories recalled by odors were significantly more emotional and evocative than those recalled by the same cue presented visually or ...
[33]
Smell and memory – The Proust Phenomenon | BPS
Apr 27, 2021 · We now know that odours are better cues in triggering autobiographical memories than other stimuli. This is because of the direct connections olfaction has ...
[34]
An experimental characterization of odor-evoked memories in humans
The results demonstrated that, consistent with folklore, odor-evoked memories tended to be highly emotional, vivid, specific, rare and relatively old. The most ...Missing: recall | Show results with:recall
[35]
(PDF) The Emotional Distinctiveness of Odor-evoked Memories
Aug 10, 2025 · Odor-evoked memories were found to be more emotional than verbally cued memories on a variety of measures. Moreover, if the cue for recall (odor ...
[36]
Cross-Cultural Color-Odor Associations - PMC - NIH
Jul 9, 2014 · Culturally-specific emotional experiences with particular odors may explain differences in how pleasant the odors are perceived to be; ...
[37]
(PDF) Psychological Effects of Multisensory VR Exposure-Based ...
Aug 6, 2025 · The objective of this study was to test the immersion effect and psychological effects of a new multisensory VR exposure-based autobiographical memory ...
[38]
Olfactory Virtual Reality: A New Frontier in the Treatment and Prevention of Posttraumatic Stress Disorder
Summary of each segment:
[39]
Unique salience of maternal breast odors for newborn infants
Beginning within minutes after birth, maternal breast odors elicit preferential head orientation by neonates and help guide them to the nipple.
[40]
Chemical communication and mother-infant recognition - PMC - NIH
Human infants are responsive to maternal odors beginning shortly after birth. They show an attraction to amniotic fluid odor that may reflect fetal exposure to ...Pheromones Or Signature... · Mother-Child Identification · Volatile Compounds Behind...
[41]
How early olfactory experiences influence brain development in mice
Environmental odor stimuli promote the growth of projection-neuron dendrites and the formation of synapses within glomeruli (Liu et al., 2016; Inoue et al., ...
[42]
Olfactory learning in the rat immediately after birth - PMC - NIH
An infant rat's chance of survival is increased when it remains close to the nest. Early olfactory learning supports such adaptive behavior.
[43]
The Effect of Breast Milk Odor on Transition Time From Gavage to ...
Dec 23, 2014 · We have shown that preterm infants exposed to breast milk odor during gavage feeding made a transition to oral feeding sooner and were ...
[44]
Group and kin recognition via olfactory cues in chimpanzees (Pan ...
Our results suggest that chimpanzees use olfactory cues to obtain information about social relationships and fill a gap in our understanding of primate ...
[45]
Multiple Memory Traces for Olfactory Reward Learning in Drosophila
Oct 10, 2007 · These results imply that at least two memory traces for odor–sugar association may exist in Drosophila: one in the first- and another in the ...Introduction · Discussion · Multiple Memory Traces And...
[46]
Remembering Components of Food in Drosophila - Frontiers
Feb 18, 2016 · Insects can learn to associate sensory cues with components of food, such as sugars, amino acids, water, salt, alcohol, toxins and pathogens.
[47]
Experimental evidence of memory-based foraging decisions ... - PNAS
To date, little research has been conducted on the cognitive mechanisms—memory or perception—by which large mammals make foraging decisions in nature.
[48]
Parallel Reinforcement Pathways for Conditioned Food Aversions in ...
Dec 21, 2010 · Conditioned olfactory aversions to toxins based on a preingestive mechanism require the brain to form an association between two types of ...
[49]
Olfactory modulation by dopamine in the context of aversive learning
Because dopamine release in higher brain areas mediates aversive learning we developed a naturalistic, ecologically inspired aversive learning paradigm in which ...
[50]
Detection and avoidance of a carnivore odor by prey - PNAS
Jun 20, 2011 · Here, we identified a chemical produced by predators that activates a mouse olfactory receptor and produces an innate behavioral response.
[51]
Genetic/Neural Architecture for Vertebrate Odor Perception
Oct 24, 2017 · Evolution sculpts the olfactory nervous system in response to the unique sensory challenges facing each species.
[52]
Loss of Olfactory Receptor Function in Hominin Evolution - PMC - NIH
Jan 2, 2014 · An analysis of the 3D structure of the Neanderthal cranium shows a significant size difference between the olfactory bulb of modern humans and ...
[53]
But how does it smell? An investigation of olfactory bulb size among ...
Mar 10, 2025 · Here, we examine the relative size of the olfactory bulbs in cranial endocasts of living and fossil primates and their kin (Euarchontoglires)
[54]
https://www.pnas.org/doi/10.1073/pnas.1103317108
[55]
[PDF] The ontogeny of kin-recognition mechanisms in ... - Jill M. Mateo
Feb 27, 2017 · Odor cues mediate so- cial recognition in Belding's ground squirrels, Urocitellus beldingi, and can be used to discriminate both individuals and ...
[56]
Scent marking behavior as an odorant communication in mice - PMC
Mice deposit scent marks toward conspecifics to maintain their social relationships, and inhibit scent marking in a context where natural predator, cat odor is ...
[57]
MHC-correlated mate choice in humans: A review - ScienceDirect.com
Here we critically review studies on MHC-associated mate choice in humans. These are based on three broadly different aspects: (1) odor preferences, (2) facial ...Review · Introduction · Mhc And Facial...
[58]
MHC-dependent mate preferences in humans - Journals
Here we show that the MHC influences both body odours and body odour preferences in humans, and that the women's preferences depend on their hormonal status.
[59]
Major histocompatibility complex-associated odour preferences and ...
MHC-associated mate preferences have also been tested in humans. An initial study reported preferences for male odours of MHC-dissimilar individuals in ...(a). Genomic Studies · (d). Mate Selection · 4. Discussion
[60]
Review The neurobiology of pair bonding: Insights from a socially ...
Research in this socially monogamous rodent has provided valuable insight into the neurobiological mechanisms that regulate pair bonding behaviors.
[61]
The Neurobiology of Love and Pair Bonding from Human and ...
Jun 12, 2023 · Here, we review current rodent and human literature on bonding ... For rodents specifically, olfactory and auditory cues from an opposite ...
[62]
The Human Sense of Smell: Are We Better Than We Think? - PMC
Recent genetic studies show a decline in the number of functional olfactory receptor genes through primate evolution to humans. Human evolution was ...
[63]
Olfactory Dysfunction as an Early Biomarker in Parkinson's Disease
Olfactory impairment is one of the most common and best characterized non-motor features in PD with a prevalence of 50%–90%. Hyposmia is often one of the first ...
[64]
Olfactory dysfunction in dementia - PMC - NIH
Indeed, olfactory dysfunction has a considerable prevalence in dementia, with estimated numbers as high as 100% in Alzheimer's disease (AD)[6], 90% in ...
[65]
Meta-Analysis of Olfactory Function in Schizophrenia, First-Degree ...
May 2, 2013 · The authors found that schizophrenia and psychosis were strongly associated with olfactory dysfunction in both database cohorts (OR = 6.2; 95% ...Missing: prevalence | Show results with:prevalence
[66]
Olfactory Dysfunction in Mental Illness - PMC
Jan 25, 2023 · Odour discrimination has been also described impaired in schizophrenia patients, although only a small number of studies investigated odour ...
[67]
The human olfactory system in two proteinopathies: Alzheimer's and ...
Jun 3, 2020 · Both disorders present a long prodromal period, characterized by preclinical signs including hyposmia. Interestingly, the olfactory system, ...
[68]
Olfaction and neurocognition after COVID-19: a scoping review
Jun 28, 2023 · Of these, seven studies found that those with olfactory dysfunction had significantly worse memory than those without olfactory dysfunction ...
[69]
Neurobehavioral Analysis to Assess Olfactory and Motor ... - PubMed
Olfactory dysfunction is the most common nonmotor PD sign in at least 90% of cases, frequently occurring 5-10 years before motor disturbances.
[70]
https://pmc.ncbi.nlm.nih.gov/articles/PMC9875195/
[71]
Updated Percentiles for the University of Pennsylvania Smell ...
The University of Pennsylvania Smell Identification Test (UPSIT) is commonly used to assess olfaction and screen for early detection of disorders including ...
[72]
Olfactory Testing for Alzheimer's Diagnosis - - Practical Neurology
The UPSIT smell test, which costs $28.95, has a specificity of 64 percent if the smell test score is less than 34 and there is lack of smell awareness. This ...
[73]
olfactory performance assessed by the combined testing of odor ...
'Sniffin' Sticks' is a test of nasal chemosensory performance using pen-like devices, assessing odor threshold, discrimination, and identification.
[74]
How to assess olfactory performance with the Sniffin' Sticks test
The Sniffin' Sticks test uses 3 subtests (threshold, identification, discrimination) with odorant-impregnated pens to assess olfactory performance.
[75]
Normative data for the "Sniffin' Sticks" including tests of odor ...
"Sniffin' Sticks" tests nasal chemosensory function using pen-like devices for odor threshold, discrimination, and identification. Results are given as a TDI ...
[76]
Updated Sniffin' Sticks normative data based on an extended ... - NIH
Dec 15, 2018 · We examined the effects of sex (female vs male) and age (groups A–I) on TDI scores by means of analysis of variance (ANOVA).Missing: protocols | Show results with:protocols
[77]
Activation and habituation in olfaction--an fMRI study - PubMed
This study investigated human BOLD responses in primary and higher order olfactory cortices following presentation of short- and long-duration odorant stimuli.Missing: paradigms EEG implicit memory
[78]
(PDF) Odor habituation can modulate very early olfactory event ...
Aug 6, 2025 · In the habituation phase, participants were exposed to the same vanilla odour in a block of 20 trials. They rated the perceived odour ...
[79]
Normative data for the "Sniffin' Sticks" including tests of odor ...
It consists of tests for odor threshold, discrimination, and identification. Previous work established its test-retest reliability and validity. Results of ...<|separator|>
[80]
Assessing modified olfactory training to prevent cognitive decline in ...
... scores yields the TDI score, with a maximum of 48 points. The TDI score assesses overall olfactory function. A score≤30.75 indicates olfactory dysfunction.
[81]
Olfactory Dysfunction After SARS-CoV-2 Infection in the RECOVER ...
Sep 25, 2025 · This cohort study examines the presence, severity, and patterns of olfactory dysfunction in US adults with and without prior SARS-CoV-2 ...
[82]
Prevalence & Recovery of COVID-19 Smell Loss: COVORTS Study
Prevalence of olfactory dysfunction remains high up to 15 months after infection, and recovery rate is low.
[83]
Development of a Digital Olfactory Function Test: A Preliminary Study
Jan 2, 2024 · We recently developed a digital olfactory function test called Digitalscent (DIGITAL SCENT), which is a kiosk-type device with an integrated hardware system.
[84]
The AROMHA brain health test is a remote olfactory assessment to ...
Mar 24, 2025 · We developed the digital remote AROMHA Brain Health Test (ABHT), an at-home odor identification, discrimination, memory, and intensity assessment.
[85]
Intensive Olfactory Training in Post-COVID-19 Patients
In our study, 81.3% of participants reported that their sense of smell improved: 84.6% in the AOT group and 79.6% in the COT group (P = .763). In all groups, ...
[86]
Effects of olfactory training on posttraumatic olfactory dysfunction: a ...
Jan 24, 2021 · Olfactory training can improve the olfaction in patients with posttraumatic olfactory dysfunction within 8 months with a recovery rate of 36.31%.
[87]
Olfactory Training for Post-COVID-19 Olfactory Dysfunction: A Meta ...
Sep 18, 2025 · Combination therapies were associated with higher recovery rates (RR = 1.65, 95% CI: 1.13–2.42, p = 0.01). Conclusions: Olfactory training is an ...
[88]
Cross-cultural modification of the University of Pennsylvania smell ...
Culture-familiar odors were chosen and the kit was translated into Arabic for the study. The test was modified 3 times in 4 versions. Following this, a random ...
[89]
Alpha-Synuclein Targeting Therapeutics for Parkinson's Disease ...
Rapamycin, an FDA approved drug, increases autophagy levels by inhibiting the mTOR pathway and can reduce dopaminergic cell death, decrease levels of ...
[90]
The olfactory deficits of depressed patients are restored after ...
Oct 22, 2020 · The olfactory signature of MDE is restored after venlafaxine treatment. This olfaction improvement is mediated by depression improvement.
[91]
Treatment of 95 post-Covid patients with SSRIs | Scientific Reports
Nov 2, 2023 · This study used an exploratory questionnaire and found that two-thirds of patients had a reasonably good to strong response on SSRIs, over a quarter of ...
[92]
Exploring the effects of olfactory VR on visuospatial memory and ...
Mar 28, 2025 · By integrating olfactory cues, VR-based cognitive training can better replicate everyday sensory experiences, enhancing ecological validity and ...
[93]
Effects of intranasal neural stem cells transplantation on olfactory ...
May 16, 2025 · Our study demonstrated that NSC transplantation contributes to the regeneration of the olfactory epithelium and enhances functional recovery in ...
[94]
Assessing modified olfactory training to prevent cognitive decline in ...
Aug 13, 2025 · Initial studies indicate that modified olfactory training (MOT) may yield superior outcomes compared to COT. Thus, this study aims to evaluate ...
[95]
Aging and Olfactory Training: A Scoping Review - Oxford Academic
Longitudinal follow-ups in studies with OT may provide an opportunity to gain insight into the relationship between OT and improved outcomes and variables ...